: The first specific aim involves the synthesis of estrogens that are conjugated to platinum. Several representative estrogens, both ER-agonists and ER-antagonists, will be conjugated to platinum by standard techniques of organic synthesis. The project's second specific aim is to identify the most promising platinated estrogens by in vitro measurement of their affinity for ER. Any platinated estrogen whose affinity for ER is at least 20 percent as high as estradiol's affinity will be considered a promising candidate for in vivo evaluation of antitumor activity. The third specific aim of the project is to measure the effect of the most promising platinated estrogens on the proliferation of breast tumor cells.